perf(foundation): find pcap distributions without parsing METADATA (#2110)

pcapkit/foundation/engines/_pcap_backend.py

@@ -132,16 +132,23 @@ def installed_distributions() -> 'tuple[str, ...]':
     only way to see the distribution the import did *not* resolve to -- which is
     exactly the collision worth reporting.
 
+    Only the existence of a ``dist-info`` is asked about, through
+    :func:`importlib.metadata.distributions` rather than
+    :func:`importlib.metadata.distribution`. From Python 3.15 the latter parses
+    every candidate's ``METADATA`` file to prefer one that has metadata, and since
+    this runs on every ``pcap_ct`` extraction, that parse made the engine about
+    2.3x slower per packet (:issue:`2110`). Either call finds a distribution
+    exactly when the other does.
+
     Returns:
         The installed subset of :data:`DISTRIBUTIONS`, in that order.
 
     """
     found = []  # type: list[str]
     for name in DISTRIBUTIONS:
         try:
-            importlib.metadata.distribution(name)
-        except importlib.metadata.PackageNotFoundError:
-            continue
+            if next(iter(importlib.metadata.distributions(name=name)), None) is None:
+                continue
         except Exception:  # pylint: disable=broad-except
             # A corrupt or unreadable ``dist-info`` is not a reason to fail an
             # extraction. Detection is a courtesy; treat "cannot tell" as "not
@@ -187,7 +194,8 @@ def probe() -> 'Probe':
     """Import |pcap|_ and report what was found.
 
     Not cached. The cost after a successful first call is a :data:`sys.modules`
-    lookup, and a cache would make the answer depend on when it was first asked --
+    lookup plus the ``dist-info`` scan in :func:`installed_distributions`, and a
+    cache would make the answer depend on when it was first asked --
     which the tests, and anything that manipulates :data:`sys.path`, would have to
     work around. It is idempotent instead, via :func:`_purge`.
 

tests/foundation/engines/test_pcap_backend.py

@@ -159,12 +159,10 @@ def installed(self, present):
 
         from pcapkit.foundation.engines import _pcap_backend
 
-        def distribution(name):
-            if name in present:
-                return mock.Mock(version='stub')
-            raise importlib.metadata.PackageNotFoundError(name)
+        def distributions(*, name):
+            return iter([mock.Mock(version='stub')] if name in present else [])
 
-        with mock.patch('importlib.metadata.distribution', side_effect=distribution):
+        with mock.patch('importlib.metadata.distributions', side_effect=distributions):
             return _pcap_backend.installed_distributions()
 
     def test_reports_only_what_is_installed_in_a_fixed_order(self) -> None:
@@ -180,9 +178,30 @@ def test_an_unreadable_dist_info_is_treated_as_absent_not_fatal(self) -> None:
 
         # Detection is a courtesy; a corrupt ``dist-info`` must not be able to
         # fail an extraction.
-        with mock.patch('importlib.metadata.distribution', side_effect=ValueError('corrupt')):
+        with mock.patch('importlib.metadata.distributions', side_effect=ValueError('corrupt')):
             self.assertEqual(_pcap_backend.installed_distributions(), ())
 
+    def test_finds_a_dist_info_without_reading_its_metadata(self) -> None:
+        import importlib.metadata
+        import os
+        import sys
+        import tempfile
+
+        from pcapkit.foundation.engines import _pcap_backend
+
+        # From Python 3.15 ``importlib.metadata.distribution`` parses every
+        # candidate's METADATA, which ran on each pcap_ct extraction (#2110).
+        # Existence is all that is asked, so a METADATA read is now an error.
+        def unread(self, filename):
+            raise AssertionError(f'read {filename} of {self}')
+
+        with tempfile.TemporaryDirectory() as site:
+            os.mkdir(os.path.join(site, 'pcap_ct-1.3.0b3.dist-info'))
+            with mock.patch.object(sys, 'path', [site, *sys.path]), \
+                    mock.patch.object(importlib.metadata.PathDistribution, 'read_text', unread), \
+                    mock.patch('importlib.metadata.distribution', side_effect=AssertionError):
+                self.assertIn('pcap-ct', _pcap_backend.installed_distributions())
+
 
 @unittest.skipUnless(HAS_RUNTIME, 'runtime dependencies not installed')
 class ReasonTests(unittest.TestCase):